In the past, thermoforming certain articles, such as vehicle seat cushions and cushion covers, which required a deep undercut to be formed ended up causing excessive thinning of the cover. When this happens, the number of defective rejects can increase and become unacceptably high. Even worse, where such articles with excessive thinning caused during thermoforming are not found defective, these articles can prematurely wear out, fail or otherwise perform unacceptably.
In one prior art method of making a foam-in-place seat cushion with a thermoformed vinyl cover, a deep undercut formed in the foam mold used to first thermoform the cover in the mold before pouring foam into the three dimensionally contoured recessed cover producing a foam-in-place seat cushion. Unfortunately, as previously indicated, the deep undercut formed in the foam mold causes vinyl seat cover material to become thinner during application of vacuum and heat during thermoforming such that excessive thinning of the cover in the undercut region can occur. Since foam is poured into the seat cushion cavity formed by the thermoformed cushion cover while vacuum is applied to maintain the shape of the cover, it is very difficult if not virtually impossible to detect whether excessive thinning of the cover has occurred prior to pouring foam into the cavity to produce the foam-in-place cover.
Where excessive thinning of the cover has occurred during thermoforming, the excessively thinned portion of the cover is subjected to premature wear, tearing, and failure, all of which is undesirable. Even where excessive thinning is detected, it frequently only is detected at an inspection stage that occurs after the foam has been poured in place and cured thereby undesirably wasting time, money and resources.
What is needed is a method and apparatus for thermoforming such articles, including seat covers, in a manner that achieves the desired look that requires use of a mold with deep undercuts but which does not cause thinning of the thermoformed material in the region of the deep undercut.